Fold cross-make and build-make into configure

[Richard Kenner]

    Run-time libraries ought to be in top-level directories parallel to "gcc".

Yes, they probably should, but that will make the initial build on a system
trickier because part of the library is needed to link the compiler.

    How (if at all) does the present Ada run-time build work with
    multilibs? 

It doesn't.

    If there are problems with dependencies of the Ada front end on its
    library, perhaps bootstrap needs to be moved out to top level first.

The problem isn't "dependencies" in the normal sense because the front
end can (and perhaps should) use the last-built library, but the issue
I raised above, that doing so adds to what you need around to do an initial
build of Ada on s syetsm.